In at least one known CT system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the "imaging plane". The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a "view". A "scan" of the object comprises a set of views made at different gantry angles during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts that attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
With respect to reformatted dental scans, the reformation is performed across slices along a pre-defined curved line, in a similar manner as multi-planar reformation. A pronounced "stair case" artifact may appear in the reformatted image. This artifact is caused by patient motion and/or by irregular table motion. Increasing the amount of motion suppression in the tomographic reconstruction reduces the motion effect. For example, and for motion suppression, the projection data set is first multiplied by a set of "underscan" weighing factors before the reconstruction process. Since the underscan weighting reduces the contribution at the beginning and end of the projection data set, patient motion related artifacts can be substantially reduced.
The underscan weighting, however, reduces the contribution from the views that are closest to the projections used in the generation of the neighboring slices. As a result, although the motion effect within each tomographic slice is reduced, the discontinuity between slices is increased.
It would be desirable to provide a correction algorithm which is effective in correcting images for stair case type artifacts in dental scans. It also would be desirable to provide such an algorithm which enables reducing the motion effect within each tomographic slice without increasing the discontinuity between slices.